The present invention relates generally to a method for manufacturing a grid for gating a stream of charged particles. The dimensions of the grid elements determine the spatial extent of fields perpendicular to the plane of the grid, such that finer mesh grids have improved optical properties.
Certain types of particle measurement instruments, such as ion mobility spectrometers, make use of a gating device for turning on and off a flowing stream of ions or other charged particles. This is accomplished by disposing a conducting grid within the path of the ions. Alternately energizing or de-energizing the grid then respectively deflects the ions or allows them to flow undeflected.
The most common method for implementing such grid uses an interleaved comb of wires, also referred to as a Bradbury-Nielson Gate. Such a gate consists of two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. When a zero potential is applied to the wires relative to the energy of the charged particles, the trajectory of the charged particle beam is not deflected by the gate. To deflect the beam, bias potentials of equal magnitude and opposite polarity are applied to the two sets of wires. This deflection produces two separate beams, each of whose intensity maximum makes a corresponding angle, alpha, with respect to the path of the un-deflected beam.
One approach to manufacturing a gating grid is disclosed in U.S. Pat. No. 4,150,319 issued to Nowak, et al. In this technique, a ring-shaped frame is fabricated from a ceramic or other suitable high temperature material. The two sets of wires are wound or laced on the frame. Each set of wires is actually a single, continuous wire strand that is laced back and forth between two concentric series of through-holes that are accurately drilled around the periphery of the frame.
Another technique for manufacturing such a gate is described in U.S. Pat. No. 5,465,480 issued to Karl, et al. In this approach, the gating grid elements are produced from a thin metal foil by cutting or etching the foil to produce the grid structure. The gird elements are connected to side electrodes in a desired pattern to produce the two sets of wires. The foil grid structure is made mechanically stable by attaching it to an insulating support member. After the then-rigid grid structure is affixed to the insulating support member, the grid elements are selectively severed from the side electrodes to form the interdigitated grid.
Yet another approach for manufacturing such a grid is described in the paper by Kimmel, J. R., et al., entitled “Novel Method for the Production of Finely Spaced Bradbury-Nielson Gates,” in Review of Scientific Instruments, Vol. 72, No. 12, December 2001, pp. 4354-4357. In this method, a guide is first manufactured out of a polymer block. The guide has a series of evenly spaced parallel grooves. A hole is drilled through the center of the polymer block; this hole eventually carries the ion beam. The machined polymer block is mounted on an insulated face of an H-shaped portion of a single sided, copper clad circuit board, with the grooves running from top to bottom of the H. The polymer-to-copper clad contacts are then fixed using an epoxy. Two small portions of the single sided copper clad board are fixed on the bottom side of the polymer in the region where the block extends over the center bar of the H-shaped copper frame.
A hand cranked, rotating screw is then used as a weaving instrument. In particular, a gold-plated tungsten wire runs from a spool over a directing screw and is coupled to the hand cranked screw by a belt. The loose end of the wire is then fixed such as by using an epoxy. A weight is hung from the wire between the directing screw and the spool in order to provide a constant tension on the wire.
A still further method was described in U.S. Patent Publication No. US-2003-0048059-A1, as published on Mar. 13, 2003. In that method, the grid is fabricated using a substrate formed of a ceramic, such as alumina. The substrate serves as a rectangular frame for a grid of uniformly spaced wires stretched across a center rectangular hole. On either side of the frame, nearest the hole, a line of contact pads are formed. Adjacent the line of contact pads, on the outboard side thereof, are formed a pair of bus bars. The contact pads and bus bars provide a way to connect the wires into the desired two separate wire sets of alternating potential. Specifically, a metal film is deposited on the surface of both sides of the ceramic through vacuum evaporation of gold, using chrome as an adhesion layer, for example. The metal film is then patterned on the front side to form the conducting elements on either side of the hole. The desired metalization pattern can be defined by a photo-resist and chemical-etch process, a lift-off process, or by using a physical mask during an evaporation. In a next sequence of steps, individual grid wires are attached to the fabricated frame. In this process, a spool of wire is provided that will serve as the grid wires, with a tensioner arrangement provided to place constant tension on the wire as the wires are attached to the substrate.